This invention relates generally to gas turbine engines, and more particularly to apparatus for mounting shrouds made of a low-ductility material in the turbine sections of such engines.
A typical gas turbine engine includes one or more turbine rotors which extract energy from the primary gas flow. Each rotor comprises an annular array of blades or buckets carried by a rotating disk. The flowpath through the rotor is defined in part by a shroud, which is a stationary structure which circumscribes the tips of the blades or buckets. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life. Typically, the air used for cooling is extracted (bled) from the compressor. Bleed air usage negatively impacts specific fuel consumption (“SFC”) and should generally be minimized.
It has been proposed to replace metallic shroud structures with materials having better high-temperature capabilities, such as ceramic matrix composites (CMCs). These materials have unique mechanical properties that must be considered during design and application of an article such as a shroud segment. When compared with metallic materials, CMC materials have relatively low tensile ductility or low strain to failure, and a low coefficient of thermal expansion (“CTE”).
One type of segmented CMC shroud incorporates a rectangular “box” design eliminating the conventional shroud hangers which are used to mount prior art metallic turbine shrouds. Rectangular box shrouds may require tight mechanical clamping against an outer casing structure. This can lead to problems if the frictional loading from clamping is larger than the axial load on the shroud, because the shroud needs to stay in contact with an axial stop to maintain proper sealing. For this to happen the shroud must be able to slide axially. This makes the clamped design potentially dependent on frictional forces which can be inconsistent.
Accordingly, there is a need for a CMC shroud mounting structure which does not rely on frictional clamping forces or concentrated fastener loads.